Conventionally, as a method of realizing improvement of fuel efficiency of automobiles, weight reduction of various parts thereof has been pursued. For example, aluminum alloys have started to be used instead of cast iron as a material for engine blocks, and magnesium alloys have started to be used instead of steel as a material for engine covers and oil pans.
In recent years, in terms of weight reduction of automobiles, use of a material made of an aluminum alloy in an elastic member of, for example, a suspension or the like, has been considered. As such aluminum alloys, 6000 series aluminum alloys have been disclosed (see, for example, Patent Literature 1).
As aluminum alloys higher in strength than the above mentioned 6000 series aluminum alloys, 7000 series aluminum alloys have been known. If a 7000 series aluminum alloy is used, an elastic member higher in strength than a bolt made of a 6000 series aluminum alloy is able to be made.